Killed steel and method of making same

ABSTRACT

A process for producing very large killed steel forging blanks and the product thereby produced. The process consists of teeming a melt of killed steel into an ingot mold containing a sinkhead reservoir, and insulating the solidified top surface of the sinkhead with an inert material. The forged blank is characterized by the absence of ultrasonic indications of nonmetallic inclusions and internal discontinuities.

United States Patent Daws et al. 1 1 Sept. 5, 1972 [54] KILLED STEEL AND METHOD OF 3,024,507 3/1962 Gero ..164/65 MAKING SAME 3,414,042 12/1968 Behrens et a1. ..l64/122 X I 42 m1 Thomas Bethlehem; 53? 326 251323 322???? 533/53? Center Valley 2,390,500 12/1945 Charman et a1. .249/201 both of Pa. I [73] Assigneez Bethlehem Steel Corporation FOREIGN PATENTS OR APPLICATIONS 1,125,141 7/1956 France. ..164/136 [22] July 1969 840,608 7/1960 Great Britain ..164/61 [21] Appl. No.: 843,228 834,909 5/ 1960 Great Britain ..164/65 52 US. Cl. ..164/65, 164/123, 249/201, jg ifm f ffv f gf g A K 51 1m, 01. ..B22d 7/10, B22d 27/04, B22d 27/16 mph] 0 eefe [58] Field of Search ..l64/123, 122, 125, 61, 65; [57] ABSTRACT A process for producing very large killed steel forging [56] References Cited blanlcs and the product thereby produced. The process consists of teeming a melt of killed steel into an ingot UNITED STATES PATENTS mold containing a sinkhead reservoir, and insulating the solidified top surface of the sinkhead with an inert 3072985 1,1963 Ahmansson z: material. The forged blank is characterized by the 2462256 2,1949 Charman et absence of ultrasonic indications of non-metallic in- Bumbaugh et al. x clusions and internal discontinuities. 298,642 5/1884 Wellman ..l64/123 2,462,255 2/ 1949 Charman et al. ..l64/ 123 3 Claims, No Drawings KILLED STEEL AND METHOD OF MAKING SAME BACKGROUND OF THE INVENTION This invention relates to killed steel. More particularly this invention relates to a method of making killed steel particularly adapted for the production of very large forgings having minimal non-metallic inclusions and internal discontinuities.

As is well known in the art, killed steels may be melted in any conventional metallurgical furnace, for example, an open-hearth, electric, or basic oxygen furnace. The steel may be killed in the furnace or in the ladle by adding a deoxidizing agent thereto. The preferred deoxidizing agent for these very clean steels is silicon. Deoxidizing with aluminum is avoided because aluminum oxide may cause sonic indications in the finished product.

The steel is teemed into big-end up ingot molds equipped with a hot top to form a reservoir for molten metal which feeds down into the solidifying metal in the mold to reduce the shrinkage cavity associated with solidification. The prior art teaches the use of exothermic insulation, placed on the molten metal of the sinkhead, to keep this reservoir molten. Upon ignition the exothermic material decomposes, forming a blanket over the sinkhead and keeping the metal below in a molten state. However, the products of decomposition, normally aluminates and silicates, form non-metallic inclusions. These inclusions feed downwardly into the body of the solidifying ingot and are entrapped in the ingot after solidification. It is therefore necessary to crop a large portion of the bottom of the ingot to rid the ingot of a major portion of these inclusions. As a result 35 a large portion of the ingot is lost.

SUMMARY OF THE INVENTION The primary object this invention is to provide a killed hypoeutectoid steel forging blank which is characterized throughout by the absence of indications of non-metallic inclusions and internal discontinuities when sonic tested.

This object is accomplished by preparing a heat of steel in a conventional manner, teeming the molten metal into an ingot mold equipped with a hot top,allowing the sinkhead surface to solidify, insulating-this solidified surface in such a manner as to prevent it from remelting and permit contamination of the molten metal by the insulation, allowing the metal in the ingot mold to solidify in a normal manner, forging the ingot into a blank and ultrasonically testing the blank.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to this invention the metal to be teemed is refined in an electric furnace. The metal may be deoxidized either in the furnace or in the ladleafter tapping, by adding a combination of silicon and manganese thereto. The melt is teemed into big-end up ingot molds, equipped with hot tops. Teeming may be con- 5 ducted in air or under vacuum conditions.

The teemed killed steel ingot is typified by the absence of gas evolution. The top surface of the sinkhead reservoir begins to solidify thereby sealing the solidifying metal from external contamination.

The last metal to solidify is contained in the lower portion of the sinkhead..The method of this invention is dependent upon the application of inert insulation as soon as the solid metal at the top of the sinkhead has sufficient strength to support the insulation.

Exothermic insulators are not satisfactory because remelting of the solid metal at the top of the sinkhead may occur and the insulatingmaterial will contaminate the body of the ingot. To avoid contaminating the ingot an inert material is used for insulation. This material should possess the following properties: low density, a relatively high melting point and inability to react with, and melt, the hot solidified surface. As an example, the combination of a first layer of a fibrous thermal insulator such as mineral wool or slag wool and a second tion to determine what portion must'be cropped away in order to insure the quality of the product.

Non-metallic inclusions and internal discontinuities are located and detected by a technique which makes use of the echo principle. This basic principle is well known to-those skilled in the art. The text book, Ultrasonics, by Benson Carlin published by McGraw-Hill Book Company, Inc., 1949, describes in detail the ultrasonic test procedure practiced by the inventors.

The testing frequency range normally employed is 40 0.5-5.0 megacycles. For optimum results the correct operating frequency was found to be 1.0 megacycles.

Experience has shown that the cathode ray tube should be calibrated so that the amplitude of the back reflection is 1% inch sweep to peak. This amplifies the sine wave to a satisfactory level for measuring nonthe back reflection.

Compensating for the rough, irregular surface and the microstructure of the forging blank ultrasonic indications greater than ten percent of the amplitude of the back reflection are considered objectionable.

A comparison of prior art forging blanks and forging blanks produced in accordance with the instant invention is as follows:

FORGING BLANKS PRODUCED IN ACCORDANCE WITH THE PRIOR ART Forging Top crop Bottom Stool Forging blank, sinkhead, Percent crop, Percent crop, Percent 0. wt. lbs. wt. lbs. removed wt. lbs. removed wt. lbs. removed Average. 679, 330 128,000 18.90 39, 520 5.83 J, 330 1.37

FORGING BLANKS PRODUCED IN ACCORDANCE WITH THE METHOD OF THIS INVENTION Forging Top crop Bottom Stool Forging blank, sinkhead, Percent crop, Percent crop, Percent 0. wt. lbs. wt. lbs. removed wt. lbs. removed wt. lbs. removed 679, 330 128, 000 18. 90 0.00 J, 330 1. 37 679, 330 128, 000 18. 90 0 0.00 l), 330 1. 37 679, 330 128, 000 18. 90 0 0. 00 J, 330 1. 37 679, 330 128, 000 18. 90 0 0.00 J, 330 1. 37 695, 710 128, 000 18. 37 0 0.00 J, 330 1. 34 695, 730 128, 000 18.36 0 0.00 0, 330 l. 34

Average- 684, 793 128, 000 18. 75 0 0.00 9, 330 1. 36

These results show how the method of this invention eliminates the necessity for bottom cropping and generates a higher yield. From a quality standpoint the bottom portion of this novel product is comparable to the ingot body of the prior art.

Following is a specific example of producing a forging blank in accordance with this invention.

Electric furnaces are used to melt steel of the following composition:

P .25 max. 8 .25 max. Si .10/.40 Ni 2.0/5.0 Cr 2.50 max.

Va 1.00 max. Mo 1.00 max.

After this Si-killed highly alloyed melt is tapped into a furnace ladle and prior to teeming into an ingot mold the entrapped gases are removed by means of a vacuum. When the vacuum degassed molten steel rises to a predetermined level in the hot top the ladle nozzle is shut off.

After breaking the vacuum and purging the system with an inert gas the dome of the vacuum tank is removed. The top of the sinkhead is exposed to the atmosphere for a suitable interval to form the plate (approximately 30 minutes) and the sinkhead is insulated with a first layer of Zinches 4inches of fibrous thermal insulation and a second layer of 8inches 12 inches of vermiculite.

After the ingot has solidified it is removed from the ingot mold and charged into a forge furnace where it is heated uniformly to forging temperature. The ingot is then removed and forged into a blank.

This initial forging step produces an article of manufacture that can be ultrasonically tested. The as-cast ingot can not be. tested because of (1) its corrugated surface, inherited from the ingot mold, and its dendritic microstructure which is not susceptible to induction of ultrasonic impulses; and (2) internal porosity associated with solidification. If this porosity is not reduced an accurate evaluation of non-metallic inclusions and internal discontinuities could not be attained. The forged blank is now ultrasonically tested. The test results are evaluated and dictate what portion of the forged blank will be cropped off.

The process of this invention is especially adapted to produce extremely large forging blanks suitable for further processing into parts for electrical equipment such as rotors, water wheels, shafts, and spindles. These parts are manufactured as stringent cleanliness specifications.

This process has now made it possible to manufacture these parts in one piece. Heretofore, the larger shafts and spindles could only be produced by connecting two smaller component parts; now a single integral part can be manufactured.

We claim:

1. A method of casting molten killed steel into an ingot mold having a hot top casing for a sinkhead reservoir comprising:

a. teeming the steel into the ingot mold and sinkhead reservoir until an upper surface of said steel reaches a predetermined pouring height within the hot top,

. insulating the molten steel which contacts the hot top, to retard solidification in the area of contact,

. exposing the upper surface of said steel for a period of time to induce superficial solidification of said upper surface to prevent external contamination of the molten steel remaining in the sinkhead reservoir, said steps (a)(b) and (c) being carried out under vacuum,

d. insulating said superficially solidified upper surface with an inert material immediately after superficial solidification.

2. A method as recited in claim 1 wherein said insulating step comprises:

a. placing a first layer of fibrous thermal insulator over said superficially solidified up r surface and b. placing a second layer of vermlcu l e over said first layer. 3. A method as recited in claim 1 wherein the length the period in which exposing of the upper surface is performed is about thirty minutes. 

1. A method of casting molten killed steel into an ingot mold having a hot top casing for a sinkhead reservoir comprising: a. teeming the steel into the ingot mold and sinkhead reservoir until an upper surface of said steel reaches a predetermined pouring height within the hot top, b. insulating the molten steel which contacts the hot top, to retard solidification in the area of contact, c. exposing the upper surface of said steel for a period of time to induce superficial solidification of said upper surface to prevent external contamination of the molten steel remaining in the sinkhead reservoir, said steps (a)(b) and (c) being carried out under vacuum, d. insulating said superficially solidified upper surface with an inert material immediately after superficial solidification.
 2. A method as recited in claim 1 wherein said insulating step comprises: a. placing a first layer of fibrous thermal insulator over said superficially solidified upper surface; and b. placing a second layer of vermiculite over said first layer.
 3. A method as recited in claim 1 wherein the length the period in which exposing of the upper surface is performed is about thirty minutes. 